MHD activity in JET hot ion H-mode discharges

Abstract

The MHD effects observed in the hot ion H-modes, including those generated in the Preliminary Tritium Experiment, are described. Some observations were found to be similar to those in high beta regimes, while others are new and appear to be pertinent to high performance discharges only. The high performance phase is largely sawtooth-free and dominated by fishbone activity, which increases in amplitude throughout this phase. During termination of the high performance phase, the growth of a large variety of MHD activity with low mode numbers is observed. Also, edge instabilities possibly associated with much larger mode numbers are seen in D(alpha) emission. In some cases the unusual structure of two central m = n = 1 islands was found. Resistive MHD modelling indicates a nearly flat, non-monotonic q-profile. In many discharges a sawtooth collapse with an extremely fast heat-wave, immediately followed by an ELM is observed at, or shortly following, the termination of the high performance.